An electrophotographic image formation apparatus irradiates the surface of an image carrier such as a photosensitive material or electrostatic recording medium with a laser beam emitted from a laser source, thereby forming image information as an electrostatic image on the surface. At this time, the laser beam is controlled to an ON or OFF state on the basis of an ON/OFF signal to drive the laser source on the basis of image data.
The emission time of the laser beam (laser emission time) is preferably always constant for each duration of the ON/OFF signal to drive the laser source. The laser emission time is also preferably always constant for the ON/OFF signal of a single laser. In fact, even when a laser ON/OFF signal of the same time (same timing) is input to the laser source driving circuit, the rise/fall of the current varies due to a variation in rising (time constant) of the current of the circuit (laser source driving circuit) to drive the laser source. Since the response time of laser emission changes because of a variation in laser emission driving threshold or current vs. output light characteristic, the emission time of the laser beam output from the laser source cannot be constant.
Generally, the time (laser ON/OFF time) to ON/OFF-control the laser source to form a 1-dot image shortens as the resolution and velocity of the image formation apparatus increase. In an image formation apparatus with a resolution of, e.g., 600 dpi, the time is about 100 ns. If the resolution doubles to 1,200 dpi without changing the process speed, the laser ON/OFF time must be as short as 25 ns. If the velocity doubles while keeping the resolution at 600 dpi, the laser ON/OFF time shortens to 50 ns.
That is, the variation width of the laser emission time cannot be neglected as the resolution improves, i.e., the laser ON/OFF time per dot shortens. When the image formation apparatus gets higher resolution and velocity, the variation width of the laser emission time becomes nonnegligible. The size of an obtained dot may vary, or no dot can be formed at a desired position.
The relationship between the laser ON/OFF time, the current, and the light beam intensity will be described with reference to FIGS. 3A to 3C. FIG. 3A is a timing chart showing the relationship between the laser ON/OFF signal and the time (ns). FIG. 3B is a timing chart showing the relationship between the time (ns) and the current (mA) (to be also referred to as a “laser current” hereinafter) flowing to the circuit to drive the laser source. FIG. 3C is a timing chart showing the relationship between the time (ns) and the intensity (mW) of a light beam emitted from the laser source by the current shown in FIG. 3B.
When the laser ON/OFF signal is turned on, the laser current gradually increases with a gradient time constant shown in FIG. 3B. When the laser current reaches a threshold current Ith, the laser source starts emitting light, as shown in FIG. 3C.
On the other hand, when the laser ON/OFF signal is turned off (FIG. 3A), the laser current gradually decreases with a gradient time constant shown in FIG. 3B. When the laser current becomes smaller than the threshold current Ith, the laser source stops emission (FIG. 3C). When such a waveform is obtained, a laser emission time TL is defined as the half width (time to ensure the 50% intensity of the light beam) of the time—optical output waveform amplitude, as shown in FIG. 3C.
As shown in FIG. 3B, the laser current has a rise time Tr after the laser ON signal is received until the current reaches Iop and a delay time Tf from Iop to 0. The rise time Tr and delay time Tf are determined by the time constant of the circuit to drive the laser source.
A technique is used to correct the variation in laser emission time due to the rise time Tr and delay time Tf, in which the rise time Tr and delay time Tf are measured in advance during the manufacture of the image formation apparatus or during its adjustment operation before image generation, and the laser ON/OFF signal is corrected by an amount corresponding to the rise time Tr and delay time Tf.
FIGS. 4A to 4D are timing charts showing the correction. FIG. 4A is a timing chart showing the relationship between the time and the laser ON/OFF signal. FIG. 4B is a timing chart showing the relationship between the time and the corrected laser ON/OFF signal. FIG. 4C is a timing chart showing the relationship between the time and the current (mA) flowing to the circuit to drive the laser source turned on by the corrected laser ON/OFF signal. FIG. 4D is a timing chart showing the relationship between the time (ns) and the intensity (mW) of a light beam emitted from the laser source by the current shown in FIG. 4C.
In this example, correction is done to make an ON time Tw (FIG. 4A) of the laser ON/OFF signal equal to the laser emission width TL (FIG. 4D).
First, the rise time Tr is measured. On the basis of the rise time Tr, the laser ON signal shown in FIG. 4A is corrected so that it advances by a time Trcor, as shown in FIG. 4B. Similarly, the delay time Tf is measured in advance. On the basis of the delay time Tf, the laser OFF signal shown in FIG. 4A is corrected so that it advances by a time Tfcor, as shown in FIG. 4B. With this processing, Tw and TL are corrected to the same width.
Another method is also disclosed in which the OFF timing of the ON/OFF signal is delayed by the time Trcor to correct the laser emission delay corresponding to the rise time Tr. Similarly, the ON timing of the ON/OFF signal is delayed by Tfcor to correct the laser OFF delay corresponding to the delay time Tf. With this processing, Tw and TL are corrected to the same width. Still another technique is disclosed in which correction is executed to hold a predetermined relationship between Tr and Trcor and between Tf and Tfcor, thereby making the laser ON/OFF signal and laser emission time have a predetermined relationship.
An example of the above-described prior arts is disclosed in patent reference (Japanese Patent Laid-Open No. 2002-154236).
However, in the above-described prior art, the correction amount of the laser rise time Tr and fall time Tf is always constant independently of image data. Hence, as shown in FIGS. 5A to 5D and FIGS. 6A to 6D, the laser emission time varies due to variations in Tr and Tf.
FIGS. 5A to 5D are timing charts showing variations in rise time of the current flowing to the circuit to drive the laser source at the laser lighting (ON) timing. FIGS. 5A and 5C show the relationship between the time (ns) and the laser ON/OFF signal. FIGS. 5B and 5D are timing charts showing the relationship between the time (ns) and the current flowing to the circuit to drive the laser source by the laser ON/OFF signal.
As shown in FIG. 5A, if a laser OFF time Toff immediately before switching from ON to OFF is long, the laser rise time Tr tends to be long. This occurs due to the influence of charges remaining in the laser driving element. This phenomenon will be described with reference to FIG. 7. Referring to FIG. 7, reference numeral 608 denotes a driving element of the output unit of a laser driving unit 606. Charges 609 in the base unit of the driving element 608 are accumulated by a circuit (not shown) of the preceding stage at the ON timing of the laser source so that a current flows between the base and emitter. Then, the current to drive the laser starts flowing to the output unit (collector) of the driving element 608. If the laser OFF time Toff is long, as shown in. FIG. 5A, the charges 609 decrease to almost zero, and the driving element is charged again from that state. Hence, the time after the driving element 608 is turned on until the current starts flowing to the circuit to drive the laser source, i.e., the rise time Tr becomes long.
On the other hand, if the ON/OFF interval is short, as shown in FIG. 5C, the charges 609 still remain, and the driving element is charged again from that state. Hence, the rise time Tr is shorter than in FIG. 5A (FIG. 5D).
The rise time Tr has been described above. The same phenomenon occurs even for the fall time Tf. FIGS. 6A to 6D are timing charts showing variations in fall time of the current flowing to the circuit to drive the laser source at the laser extinguishing (OFF) timing. FIGS. 6A and 6C show the relationship between the time and the laser ON/OFF signal. FIGS. 6B and 6D are timing charts showing the relationship between the time (ns) and the current flowing to the circuit to drive the laser source by the laser ON/OFF signal.
As shown in FIG. 6A, if an ON time Ton of the laser source before it is turned off is long, the laser current fall time Tf tends to be long. This also occurs due to the influence of charges remaining in the laser driving element 608, as in FIGS. 5A to 5D described above. As the laser ON time Ton becomes long, as shown in FIG. 6A, the residual time of the charges 609 is prolonged, and the time until the laser is turned off (extinguished), i.e., Tf becomes long.
When the ON state time is short, as shown in FIG. 6C, the influence of charges remaining in the driving element 608 is smaller than in the state shown in FIG. 6A. Hence, the time until the laser is turned off (extinguished) is shorter than in FIG. 6A (FIG. 6D).
That is, in the conventional method in which the correction amount of the laser rise time Tr and fall time Tf is always constant independently of image data, the variation in laser emission time, which occurs in accordance with the duration of the state immediately before switching the laser from OFF to ON or from ON to OFF, cannot be corrected sufficiently.
The present invention has been made to solve the above-described problem, and has as its object to provide an image formation technique which enables stable image formation without any variation in dot size or dot formation position based in image data by correcting the relationship between the laser ON/OFF signal and the actual laser emission time including the laser rise time Tr (to be also referred to as a “rise delay time” hereinafter) and fall time Tf (to be also referred to as a “fall delay time” hereinafter) to a constant relationship independently of duration of the immediately preceding state based on image data, i.e., an image formation technique capable of suppressing the variation in laser emission time by correcting the timing of the laser ON/OFF signal, thereby stabilizing the dot size or dot formation position in image formation.